Mirrors can be made by depositing chemically or by vapor deposition thin films of metals onto a substrate such as a glass surface. The metal films must be protected, however, for otherwise they lose their reflectivity within a very short period of time due to oxidation and attack by atmospheric pollutants. The glass protects one side of the film and the other side is usually overcoated with, for example, copper, and then painted to protect the metal film.
In solar energy conversion large numbers of mirrors are employed in heliostats. As this technology is rapidly developing, there are considerable additional demands on the durability and reflectivity of these mirrors. It would be desirable, for example, to use front surface mirrors rather than the usual back surface mirrors to reduce energy loss through the glass plate. Another advantage of the front surface mirror is that the substrate beneath the surface need not be of high quality plate glass. Any suitable material, even metals and plastics, could be used, permitting significant cost savings. The use of front surface mirrors, however, requires protection of the highly reflective metallic surface from oxidation and environmental pollutants by means of a transparent coating. At the present time, magnesium fluoride coatings are used on front surface mirrors, but these mirrors are very expensive.